Indirect, air-to-air heat exchangers are devices that are used to extract thermal energy from dirty heated gas and provide that thermal energy to a wide variety of diverse application such as heating clean ambient air, liquids, chemical processes, and similar uses. The source from which the extraction is made is usually waste gas of some kind, such as hot waste fumes from an industrial furnace or the like.
In general, conventional shell and tube heat exchangers utilize a series of tubes supported at their ends by what is known in the art as tube sheets. Ambient air flows through, or is forced through the tubes, and a cross flow of the hot gases, usually waste gases, is passed in a cross flow over the outside surface of the tubes to heat the air flowing through them. This is the concept of "heat exchange". It will be noted that the applicant contemplates that the cross flow can be air and the material flowing through the ceramic tubes can be hot or waste gases.
Some conventional types of heat exchangers employ metal tubes which are welded at their ends to a supporting metal tube sheet. These metal heat exchangers are subject to deterioration from chemically corrosive or abrasive particles and further, they are subject to wide latitudes of expansion under operating conditions.
Conventional heat exchangers employing ceramic components have been used in the past in these types of adverse environments. One type of heat exchanger in this category employs a sponge or matrix made of ceramic material. The particulates in the waste fumes have a tendency to plug the matrix after a period of time thereby decreasing the efficiency and, in some instances, creating a fire hazard.
Yet another type of heat exchanger employs metallic springs pushing against one end of the ceramic tube or tube sheet in an effort to provide sealing engagement between the tube and the supporting tube sheet. Systems employing metal components to seal ceramics are subject to leakage problems since metal has a different rate of expansion than ceramic. In addition, the metallic components are still subject to deterioration under the above-mentioned adverse conditions in which these types of heat exchangers may be used. Also, in the likely event of power failure, the metallic components will fail when air side cooling stops.
Most of the known heat exchanger designs employ straight sided tubes which empty into plenums formed between the supporting tube sheets and the inner wall of the external housing or casing. The plenums are designed to carry the ambient air to other zones in the internal heat exchanger construction employing another set of tubes for passing the air back through the central chamber through which the heated waste fumes flow. Thus, the heat exchangers are normally stacked or otherwise fastened together to increase the operating flow length of both the ambient air and the waste gas and the flow of the ambient air between the plenums and tubes creates a pressure loss within the system. These pressure losses must be overcome by an increase in the horsepower of the fans for moving the ambient air in order to maintain a given velocity of the ambient air flow. These pressure losses also make it difficult to operate at high pressures, and consequently, the heat exchangers of the prior art are not operated at the higher pressures, or if attempts are made to do so, there is severe leakage. These pressure losses also make it difficult to maintain an air tight seal from the ambient air side to the gas side subsystem. The resultant leakage which may occur not only decreases the flow of the ambient air, but also allows air to flow into the fumes to reduce overall heat transfer efficiency. Also, there is an acute operating temperature loss in the heat exchanger with this type of arrangement. Air Side temperatures at operation of the prior art heat exchangers range from about 800.degree. F. to about 1200.degree. F., while the temperatures permitted by the use of the heat exchanger of the instant invention can range from 800.degree. F. to 2400.degree. F. Further, the pressures at operation of the prior art heat exchangers range from 0.25 psig to 250 psig, while the pressures permitted by the use of the heat exchanger of the instant invention can range from slightly above zero psig to 15 psig. Therefore, for purposes of this invention, what is meant by "low to medium pressure" are pressures in the range of slightly above zero psig to 15 psig, and what is meant by "high temperatures" are temperatures in the range of 1800.degree. F. to 2800.degree. F.
One of the most egregious forms of inefficiency in heat exchangers occurs in the connections of the tubes to the tube sheets, wherein leakage is usually of a high volume. Further, these prior art connections are machined to decrease the tolerances and to prevent high leakage and this adds to the overall cost of such systems. In addition, the tube sheet itself is subject to expansion and when it expands, it expands in an uncontrolled manner which causes the tube sheet to move out of alignment, and thus causing more leakage. The prior art tube sheets also have a problem in that the tiles are manufactured such that they contain only one half of a tube opening in them and thus, that means many tube tiles have to be mortared together to obtain a tube sheet. Since these mortared joints microcrack under operating conditions, the more mortar joints that are used in a heat exchanger, the more leaks that occur in the tube sheets.
The heat exchangers of the prior art that are subject to many of the problems set forth above can be found in one or more of the following patents: U.S. Pat. No. 1,429,149, U.S. Pat. No. 1,974,402, U.S. Pat. No. 3,019,000, U.S. Pat. No. 3,675,710, U.S. Pat. No. 3,923,314, U.S. Pat. No. 4,018,209, U.S. Pat. No. 4,106,556, U.S. Pat. No. 4,122,894, U.S. Pat. No. 4,449,575, and U.S. Pat. No. 4,632,181, and the United Kingdom patents, 191,175, issued in January, 1923, and 2,015,146, issued in September of 1979.
One notable publication dealing with a flexible ball joint system for joining ceramic heat exchanger tubes to tube sheets is that entitled "FLEXIBLE BALL JOINT SYSTEM", dated Apr. 11, 1995 in which there is shown a flexible ball joint system sold by Sonic Environmental Systems, Inc. wherein there is shown in an exploded view, a plug, a ball seal, a collar and a ceramic tube. This assembly has a slip surface between the tube and the ball seal. When the tube slides in and out of the seal due to thermal expansion, it does not pull the ball seal against the inner surface of the inner tube sheet tile or the ceramic tube. This results in a situation in which, when the tube sheets move during operation, the ball seals do not maintain their compression and leakage occurs. Further, without an attachment to the ceramic tube, the seal cannot move, either in a linear direction, or a lateral direction, both of which are requirements in a good sealing system. Thus, there is needed a decidedly different heat exchanger to overcome the problems set forth above.